As computers and processors become more powerful, more and more signal processing is being done in the digital domain. Digital signal processing can perform complex operations to manipulate input data to approximate real world analog signals, and the operations can be performed in real time, or the digital data can be stored for future processing. Since real world signals exist as analog signals, these analog signals need to be converted to equivalent digital signals.
Analog to digital converters (ADCs) are used in many applications, such as, for example, converting analog control signals in industrial applications, audio signals in music, photographic images in digital cameras, and video images in digital video cameras. As with most circuits, there are many different types of ADCs where tradeoffs are made for different limitations. Some, such as the “flash” ADC, are relatively expensive in circuitry and layout space and, accordingly, limited in resolution since every additional bit requires doubling of the number of comparators, but very fast in conversion speed. Others, such as the ramp ADC, can be fairly simple but slow in conversion time. And as the amount of resolution increases, the conversion time will increase.
Accordingly, a particular application needs to take into account various limitations and determine which design best serves its purposes. However, picking a specific design, and possibly modifying it to improve its design, can still present certain challenges that need to be overcome.
For high resolution and high speed imaging, column parallel ADC architecture has become widely used in CMOS image sensors. The architecture may comprise single-slope ADCs that need a ramp signal to compare with the input signal. In general, the ramp signals are generated as staircases, which have undesirable limitations for a high speed operation.
This problem can be overcome by using a non-staircase, linear ramp signal. However, the slope of the ramp signal, which sets the gain and input range of the ADC, is independent of operating frequency of the ADC. Furthermore, the slope of the ramp signal is affected by the changes due to temperature variations, supply signal variations and other process variations. The gain error in the ADC can cause errors in the ADC input range, which can then cause the output image to be under-exposed or saturated. The gain errors between the different color channels can cause color distortions in the final image.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.